Sweep signal sources are well known in the art for a variety of test and measurement purposes. Typically, the frequency of a source is swept continuously or in steps between preselected start and stop frequencies. State of the art sweep signal sources operate over frequency ranges on the order of 10 megahertz to 40 gigahertz. Phase locked loop frequency synthesizers are used to provide highly accurate, stable output frequencies. Typical features include continuous or stepped sweeps, selectable start and stop frequencies, and selectable sweep times.
A frequent use of sweep signal sources is in network analyzer systems. A vector network analyzer system contains several elements. A first element is the signal source to provide a stimulus to a device under test (DUT). A second element is a signal separation network to route the stimulus to the DUT and to provide a means for sampling the energy that is reflected from or transmitted through the DUT. Also, energy from the signal that is incident upon the DUT is sampled in order to provide a reference for relative measurements. A third element is a tuned receiver to convert the resulting signals to intermediate frequencies for further processing. The magnitude and phase relationships of the original signals must be maintained through the frequency conversion to intermediate frequency to provide usable measurements. A fourth element is a detector to detect the magnitude and phase characteristics of the intermediate frequency signals, and a fifth element is a display on which to present the measurement results.
In a network analyzer system, it is necessary to synchronize the operations of the receiver to those of the source. The receiver and signal processing portions of the system take data or measurements at a number of frequencies during a sweep. The data must be precisely correlated to the frequencies at which it was taken in order to provide accurate measurements. In addition, network analyzer systems often employ markers to indicate selected frequencies on a display. In order to ensure that the markers appear at the selected frequencies, the receiver must determine when the source sweeps through that frequency. Depending on the selected start and stop frequencies, the signal source may change bands during a sweep by activating different oscillator and/or frequency multiplier configurations. When bands are changed, the sweep is temporarily stopped. The receiver must be notified when a stop sweep occurs in order to maintain synchronization.
It is customary to generate continuous frequency sweeps by applying a continuously increasing or decreasing ramp voltage to the tuning input of a voltage controlled oscillator. Although the ramp voltage can be initiated by a synchronizing signal, the ramp voltage is subject to errors from a number of sources, including timing capacitor and timing resistor tolerances, reference voltage variations, temperature variations and component aging. Furthermore, different components and different voltages are used to generate the ramp voltage, depending on the selected sweep time. When the ramp voltage is in error, the synchronizing signals have an imprecise time relationship to the ramp voltage. Consequently, operations that are synchronized to such signals are not synchronized to the sweep.
In order to reduce such errors and to more accurately synchronize operations to the frequency sweep, it is known to convert the ramp voltage into a digital pulse train using an analog-to-digital converter. Each time the ramp voltage changes by a predetermined amount, a digital pulse is generated. Thus, the synchronizing signal is generated directly from the ramp voltage. This technique is described in U.S. Pat. No. 4,641,086 issued Feb. 3, 1987 to Barr, IV et al and is implemented in the Model 8340 signal source and Model 8510 receiver, both manufactured and sold by Hewlett Packard Company. While this technique provides satisfactory performance, it is subject to errors in the analog to-digital converter which converts the ramp voltage to a digital pulse train.
Swept synthesizers sometimes generate frequency sweeps using a technique known as fractional-N sweeps. In fractional-N phase locked loops, the divider ratio is changed on the fly during a sweep, and phase errors are corrected using analog phase interpolation. The output is a phase locked analog frequency sweep. Such sweeps are generated without the use of a ramp voltage. Thus, it is highly inaccurate to synchronize the receiver by conversion of a ramp voltage to a pulse train when fractional-N sweeps are employed.
Accuracy is a factor of primary importance for sweep signal sources, as for other test instruments. In order to provide accurate measurements, it is necessary to know both the frequency and the power level at each measurement point in a sweep. The discussion hereinabove regarding synchronization of operations relates to accurate determination of frequency. It is also important to provide a known power level at each measurement point.
Although sweep signal sources are designed to provide constant output power levels during a frequency sweep, it is well known that microwave power levels vary drastically with frequency due to parasitic impedances of interconnecting cables, circuit components and the like. Typically, the power level decreases with increasing frequency. It is desirable to compensate for such power level variations as a function of frequency and to provide a constant power level, either at the output of the signal source or at a remote measurement location.
In many cases, the output of the signal source passes through cables, couplers and other components before reaching a device under test. Accurate evaluation of the device under test requires a comparison of its output with its input at specified frequencies. Thus, it is often important to provide a constant or flat power level at the remote location where the device under test is located.
It is known in the art to provide power level correction based on a fixed number of correction points in the sweep range of the instrument. When a sweep is performed, the output level is corrected in accordance with the stored corrections. However, the fixed correction points are unlikely to correspond to the measurement points during a sweep when the start and stop frequencies of a sweep are selectable. Furthermore, for short sweeps the entire sweep may fall between two of the fixed correction points. In these cases, high accuracy power leveling is not achieved.
It is a general object of the present invention to provide improved sweep frequency sources.
It is another object of the present invention to provide digitally synchronized sweep frequency sources.
It is yet another object of the present invention to provide highly accurate sweep sources.
It is a further object of the present invention to provide a sweep frequency source which generates a constant power level at a specified location during a frequency sweep.
It is yet another object of the present invention to provide methods and apparatus for interpolated swept parameter correction in sweep frequency sources.
It is still another object of the present invention to automatically provide parameter corrections at each measurement point in a selected frequency sweep.